Over many decades, the processes for forming thin skins of thermoplastic on a mold surface have evolved, driven primarily by cost and weight objectives.
Early on, electroformed nickel and nickel/copper molds were filled with a predetermined charge of liquid plastisol and rotated through a gas fired oven and into a water spray section to produce shells which were subsequently filled with urethane foam and used as vehicle arm rests and the like. Usually, the apparatus used for producing these shells was a series of multi-armed spindles that indexed between filling, heating, cooling and stripping stations.
As the demand for soft feel interior trim products for automobile interiors increased in the 1950's and 1960's, larger parts were demanded, such as instrument panels. This lead to a “slush molding” process as opposed to rotational molding, where liquid plastisol was pumped into a preheated electroformed mold to coat the mold surface (gel) and the excess dumped out before the mold indexed into the fuse and cooling stations. The large electroformed nickel tools could not be rotated in various axes due to their size nor did they need to be, as the products were becoming long and relatively flat. This led to an over and under conveyorized process which required a large number of electroformed molds (10-30) to be used in order to fill a continuously moving production line. Here, either gas fired burners or induction heating coils that resembled the contour of the mold and of the final product were used to provide heat in stations for gelling and fusing the plastic. This conveyorized process also limited the number of shapes of molds that could be processed without facility modification to basically one as the heating apparatus were shape specific.
Because of the space requirements of the conveyorized line and the cost of using many electroformed molds, a modular slush process evolved. Here a single electroformed mold was rotated around its major axis and heat and cooling supplied to it. Stainless steel tubing was welded to the back side of the electroformed mold and hot or cool heat transfer fluid was circulated through the tubes to heat and cool the mold and the liquid plastisol contained in the mold. Cleanliness was difficult to maintain in this process as thicker sections of the plastic skin, particularly drips and runs from the excess liquid plastisol being dumped out, would remain unfused and transfer to both the station operator and adjacent shells. Powder slush formations for PVC as well as other thermoplastics (TPU, TPE, TPO, ASA, etc.) next evolved to minimize waste in the slush process and produce skins of more uniform thickness. Here only a defined thickness of powder next to the mold surface melted and the unmelted powder was returned to the powder box for future use. The modular process resulted in the use of fewer molds and rapid mold changes.
A further difficulty with stainless steel tubes welded onto the electroform molds was one of shortened mold life. The heat stresses that the nickel mold was exposed to during welding of the tubes to the mold resulted in mold cracking. To solve this, other means of heating the mold were explored. Dipping the electroform into a fluidized bed (U.S. Pat. No. 4,946,663 to Takamatsu) or into a heat transfer medium was employed. Induction heating (U.S. Pat. No. 4,898,697 to Horton commonly assigned and incorporated herein by reference), and microwave heating methods have been noted. A hybrid method utilizing a robot and multiple stations is disclosed in U.S. Pat. No. 4,755,333 to Gray (commonly assigned and incorporated herein by reference).
Most popular was a modular processing apparatus where a mold box was used to enclose the back side of the nickel electroform mold and gas fire heated hot air was impinged through tubes at high velocity onto the backside of the electroformed mold to provide fast heating (or outside ambient air cooling) cycles. U.S. Pat. Nos. 6,019,390 and 6,082,989 to McNally and U.S. Pat. No. 6,013,210 to Gardner describe variations on this process. On relatively cool ambient days, cycles in the order of 4 minutes could be achieved. However, to move to the next level of cycle improvement, some of the disadvantages of this apparatus needed to be overcome. The hot air impinging in the back of the electroformed mold was at such a pressure that the nickel mold would flex and ultimately crack due to fatigue. The modular processing apparatus evolved to a large mold stand with a gas fired burner overhead and many feet of duct work supplying hot and cool air that were inefficiently heated and cooled. Ambient noise and heat pollution became issues for the station operators.
What is needed is a process that heats and cools only the mold and plastic skin material that it contains, a process that allows for the use of thin lightweight molds, an environmentally friendly process with little noise and wasted heat, and a process apparatus that can be converted from one mold/product shape to another rapidly to reduce process downtime.